5G new radio (NR) is in the process of being developed using 3GPP standard. An uplink (UL) of NR supports variable bandwidth and carrier frequency bands below 6 GHz and as well as above 6 GHz, including millimeter wave bands (e.g. 20-40 GHz and 60-80 GHz). The UL of NR supports cyclic prefix orthogonal frequency-division multiplexing (CP-OFDM) and discrete Fourier transform-spread-OFDM (DFT-S-OFDM), also referred as single-carrier frequency-division multiple access (SC-FDMA). For UL, the user equipment (UE), configured with multiple transmit antennas, involves a transmit diversity in certain channel conditions.
Generally, uplink or downlink transmit diversity (ULTD/DLTD) methods includes more than one transmit antennas at the UE or BS (base station), to improve the uplink or downlink transmission performance, i.e. reduce the transmit power, increase the coverage range, increase the data rate, or the combination of the above. The uplink transmission performance can also help improve the overall system capacity. Based on the feedback requirements, ULTD/DLTD schemes can be categorized into closed-loop (CL) and open-loop (OL) schemes.
The CL transmit diversity methods require a receiver to provide explicit feedback information about the spatial channel to assist the transmitter in choosing a transmission format over multiple transmit antennas. The OL transmit diversity methods do not have the requirement of feedback from the receiver.
For systems with transmit diversity, the received signal is a superposition of the different transmitted signals from all transmit antennas and consequently, the channel estimation becomes more complicated. In systems that employ open-loop precoding, the precoder (or the set of antenna weights) changes in at least one of time or frequency domain OFDM symbols. In such cases, the effective channel observed by the receiver undergoes variations as the precoder takes different values. There is a need for the receiver to adjust the channel estimation procedure based on precoder changes.